Inverters of that kind are used for example as independent current supplies for electrical loads which need a multiphase current, particularly a three-phase current, when no appropriate mains are at disposal. Such inverter further comprise a control unit which has a free-running time base, from which the switching command signals for the controllable switches are so derived that the different phase conductors are, for the production of an alternating voltage at the desired frequency of for example 50 Hertz or 60 Hertz, periodically connected with the different output terminals of the d.c. power supply and again separated from these.
Another possibility for use is by employment as emergency current unit which shall take over the supply of a load when the mains, which normally stand at disposal, fail and are no longer capable of delivering the required multiphase current. In that case, the above-mentioned time base initially does not run freely, but is so synchronized with the mains voltage, which is sensed by way of sensors, that the emergency current unit can take over the supply of the load as free from interruption as possible in the case of a mains failure. After failure of the mains, the time base must also here run on automatically and take care of the correct frequency and phase position of the produced voltages.
It is known for these cases of application to use as a d.c. power supply for example a battery block which in principle has only two terminals, at which the maximum crest voltage, for example 310 volts, can be taken off. The individual phase conductors are applied with the aid of thyristors to those 310 volts with alternating polarity in the required timing cycle and again separated so that, in place of a sinusoidal wave, a rectangular pulse train arises, which in case of need can be rounded off and assimilated to a sinusoidal course by choke elements, filter members or transformers. A particular problem arises in such an inverter when it is used for the supply of a not purely ohmic load and it comes to a phase displacement between current and voltage on the phase conductors by reason of inductive or capacitive loading. In such a case, during each period, time intervals occur, in which current and voltage possess opposite signs so that a current in opposite direction, thus a charging current in the case of batteries, flows through the direct voltage sources. This short period alternation between charging and discharging currents is damaging particularly for batteries and leads not only to high heat losses, but also shortens the operating life appreciably. It is therefore known to equip inverters with a plurality of coils and/or capacitors and to switch these elements in or out, in dependence on whether the load becomes capacitive or inductive in increasing or decreasing degree, in order to compensate for the phase displacement between current and voltage caused by the load.
Such arrangements are extraordinarily expensive in material and costs, have a great space requirement and are liable to faults.
An objective of the invention is to provide an inverter of the initially described kind in which phase displacements between current and voltage remain without damaging effects for the direct voltage sources for at least as long as these phase displacements do not exceed a maximum permissible phase angle.